In recent years, attention has been given to projectors using a light emitting diode (LED) as a light source. A projector of this kind has an LED, an illumination optical system into which light emitted from the LED enters, a modulating element that modulates light from the illumination optical system according to a video signal and that emits the modulated light, and a projection optical system that projects the light from the modulating element onto a screen.
With respect to the above-described projector, there is a demand for using light emitted from a light source as projection light with efficiency in order to increase the brightness of the projected image. Enabling efficient use of light emitted from a light source as projection light requires setting the etendue obtained as the product of the light emission area and the radiation angle of the light source equal to or smaller than the product of the light receiving area of the modulating element and the acceptance angle determined by the F-number of the illumination optical system.
In some cases, in the above-described projector, an element having a polarization dependence, e.g., a liquid crystal panel is used as the modulating element. In such cases, since light emitted from the LED is randomly polarized light, efficiently using light emitted from a light source as projection light requires converting random polarization into a specific polarization.
For a technique to convert random polarization into a specific polarization, a planar illumination device described in Patent Literature 1 exists. The planar illumination device has a light guide plate, a stepped microprism provided below the light guide plate, a polarization separating film provided on the light guide plate, and an upper cover provided on the polarization separating film. The polarization separating film has a structure in which a metal thin film is sandwiched between a first low-refractive-index transparent medium and a second low-refractive-index transparent medium.
In the above-described planar illumination device, light from a light source enters the light guide plate and propagates through the light guide plate while being angle-converted by the microprism. When the light is totally reflected by a first boundary i.e., the boundary between the light guide plate and the first low-refractive-index transparent medium, a surface plasmon is excited at the metal thin film by evanescent waves generated simultaneously with the reflection. When the surface plasmon is excited at the metal thin film, the process reverse to the process of excitation of the surface plasmon occurs at a second boundary i.e., the boundary between the second low-refractive-index transparent medium and the upper cover. Light is generated at the second boundary and emitted through the upper cover.
The light that excites the surface plasmon in the light striking the first boundary is only TM-polarized light having an electric field component parallel to the first boundary. Then the light generated at the second boundary is TM-polarized light, as is the light exciting the surface plasmon, since the light generated at the second boundary is produced by the process reverse to the surface plasmon excitation process. Thus, the planar illumination device can convert random polarization into a specific polarization before emitting the light.